Parts-per-billion detection of carbon monoxide: A comparison between quartz-enhanced photoacoustic and photothermal spectroscopy

We report on a comparison between two optical detection techniques, one based on a Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) detection module, where a quartz tuning fork is acoustically coupled with a pair of millimeter-sized resonator tubes; and the other one based on a Photothermal Spectroscopy (PTS) module where a Fabry-Perot interferometer acts as transducer to probe refractive index variations. When resonant optical absorption of modulated light occurs in a gas sample, QEPAS directly detects acoustic waves while PTS probes refractive index variations caused by local heating. Compact QEPAS and PTS detection modules were realized and integrated in a gas sensor system for detection of carbon monoxide (CO), targeting the fundamental band at 4.6 mu m by using a distributed-feedback quantum cascade laser. Performance was compared and ultimate detection limits up to - 6 part-per-billion (ppb) and -15 ppb were reached for QEPAS and the PTS module, respectively, using 100 s integration time and 40 mW of laser power.

Automated summary

This study compared two optical detection techniques based on QEPAS and PTS, which directly detect acoustic waves and probe refractive index variations to detect optical absorption phenomena in gases. Compact QEPAS and PTS detection modules were integrated into a gas sensor system for CO detection, achieving ultimate detection limits of -6 ppb and -15 ppb, respectively.